1. Field of the Invention
The present invention relates to an articulator that is used to reproduce human jaw movement, in particular articulation, when dental prostheses (for example, dentures etc. to replace lost teeth) are constructed.
2. Description of the Related Art
Previously known articulators will be described below, together with their limitations.
(1) What an Articulator is
The jaw performs the functions of eating and speaking etc. and, in order to achieve these functions, performs truly many different types of movement. Construction of dental prostheses is an important technique in dental treatment in order to recover lost function, and in this connection, it is important to have a good understanding and grasp of jaw movement. Thus, reproduction of jaw movement is indispensable in order to achieve good insertion of the dentures, good articulation of the upper and lower rows of dentures, and good chewing movement.
There is a considerable degree of individuality as regards the chewing movement of individuals. In order to construct good dentures, a device for reproducing jaw movement, in particular articulation, i.e. an articulator is indispensable, and a great amount of efforts is being made to develop these devices. However, an accurate full reproduction articulator corresponding faithfully to the jaw movement of individual persons has not yet appeared.
(2) Conditions for Articulation Reproduction
In reproducing (characterizing) the movement of any object, not only the jaw, the start and end positions of movement of the object should be characterized. Specifically, if the position prior to movement of an object K is K0 and its position after the movement is K1, the change of position K0.fwdarw.K1 constitutes the movement of the object K. If the jaw is thought of as a rigid body, the position of the rigid body as a whole i.e. the jaw is characterized by characterizing the positions of three points of the rigid body. Clinically, the three points A, B, C shown in FIG. 26 of the accompanying drawings are employed for these three points, but this is for the sake of convenience, and any points on the rigid body including the jaw could be used. Referring to FIG. 27, if the positions prior to and after the movement are respectively indicated as A0, B0, C0 and A1, B1, C1, the jaw movement can be reproduced by characterizing A0, A1, B0, B1 and C0, C1 (see the arrows in FIG. 27).
(3) Terms Used to Describe Occlusion
Referring to FIG. 28, in dentistry, general terms such as front plane, side plane and horizontal plane are replaced by frontal plane, sagittal plane and occlusal plane, respectively. Also, the area where movement of the jaw occurs is termed the working side, and the opposite area is termed the non-working side (or balancing side). Although in recent years the term "balancing side" has tended to become obsolete, in this characterization, this side will be referred to as the "balancing side".
The jaw consists of the maxilla and mandible and chewing is performed by articulation of the respective teeth. The maxilla is included in the skull, while the mandible is suspended by muscles and tendons from the cranium, so that only the mandible is moved. The mandible consists of the row of teeth, the body of mandible, and the condyles. In FIG. 28, the condyle at point A is the condyle on the movement side, and so is called the working side condyle, while the condyle at point B is called the balancing side condyle. In the middle of the mandible, i.e., the incisal region, the mesial point of central incisors on the left and right is called the incisal point, and the center point of the condyle is called the condylar point. Reproduction of chewing movement is characterized by the three points: left and right condylar points and the incisal point.
(4) Mandibular Movement
a) Incisal Path and Condylar Path
Mandibular movement is performed in five directions, namely, protrusively, laterally to left and right, opening and posteriorly. In this process, as shown in FIG. 29, movement of the condyle is regulated by the form of the associated joint cavity in which the condyle is accommodated. The line joining the left and right condylar points is called the "intercondylar axis". When the two condyles perform sliding movement over the surfaces of the joint cavities, the incisal points can perform rotary movement about the occasional intercondylar axis. The loci of the movement of the condylar points and the incisal point are respectively called the condylar path and incisal path.
b) Protrusive Movement
When protrusive movement is performed, as shown in FIGS. 29 and 30, the condyles move anteriorly and downwards along the shape of the joint cavities. This is on average about 30 degrees (30.degree.) with reference to the occlusal plane, and is called the inclination of sagittal condylar path during protrusive movement, abbreviated to the inclination of protrusive sagittal condylar path. The protrusive sagittal condylar path inclinations are often different between the left and right condyles. The incisal point likewise moves anteriorly and downwards, constrained by the shape of the incisors of the maxilla. The clinical reference value with respect to the occlusal plane is 10.degree..
c) Lateral Movement
As shown in FIG. 31, in the case of lateral movement (where the jaw moves to left or right), the balancing side condyle executes a large movement for what is only a slight movement of the working side condyle. Like the protrusive movement, the balancing side condyle executes anterior and downwards movement along the shape of the joint cavity; the inclination of sagittal condylar path when this movement happens is called the lateral sagittal condylar path inclination and is generally larger than the protrusive condylar path inclination, the difference of these angles being termed the Fischer angle (considered to be about 15.degree. on average).
Also, during lateral movement, the working side condyle is known to move outwards in the working direction. This outward movement is called "Bennett movement" after the name of its discoverer. Since the mandible is unitary, if the working side condyle moves outwards, the balancing side condyle is pulled inwards. As a result, the balancing side condyle moves forward, downwards and inwards. The inwards angle with respect to the sagittal direction i.e. the angle of lateral condylar path is called the "Bennett angle".
(5) Characterization of Mandibular Movement
If it can be characterized to what extent the three points (left and right condyles and incisor) move vertically, left/right and anteriorly/posteriorly, the entire mandibular movement can be characterized. Also, the direction of movement of a given point is the direction of the line of intersection of the two planes that characterize the direction of movement of that point. That is, two planes are necessary and sufficient in order to characterize the direction of movement of a given point.
Hereinbelow, the planes that characterize the three points (left and right condyles and incisor) will be described.
As already stated, the mandibular movement is effected in opening, protrusively, posteriorly, and laterally (working side and balancing side). Of these, mouth-opening movement is a movement to release occlusion, so this may be excluded from the requirements for the articulator. Also, if the characterizing plane for protrusive movement is used for posterior movement, it would be sufficient to obtain planes characterizing the three movements: protrusively, working side and balancing side. Since two planes are required for a single movement in respect of each point, reproduction of the three types of movement by each point requires six planes. Also, if the positions of the two condylar points are characterized, the left and right and anterior/posterior positions of the incisal points are relatively characterized from the two condylar points and the amount of vertical opening thereof is characterized by arbitrary choice of the technician/doctor or contact of the upper and lower plaster models, so the subject of consideration in regard to reproduction of jaw movement need be only left/right lateral movement of the condyles. Furthermore, in the case of protrusive movement, what is necessary is a single plane to characterize sagittal condylar path inclination, so in fact only five planes are required. Also, since the position of the incisal point is relatively characterized from the two condylar points after the positions of the two condylar points are characterized, and, furthermore, the setting of the amount of mouth opening at the incisal point is arbitrary, the subject of consideration in regard to reproduction of jaw movement need only be the movement of the left and right condyles. In sum, for reproduction of jaw movement in an articulator, what is necessary is only reproduction of the three movements: protrusive, balancing side and working side of the condyles. As mentioned above, for protrusive characterization, a single plane is sufficient, and it is satisfactory if movement of each of the left and right condyles can be characterized by five characterization planes, respectively.
Also, the three-dimensional movement characterization elements of the condyles are as follows:
Vertical characterization: sagittal condylar path inclination plane PA1 Internal/external characterization: Bennett plane PA1 Anterior/posterior characterization: rear wall PA1 Vertical characterization PA1 Internal/external characterization PA1 Anterior/posterior characterization PA1 vertical characterization
(6) Mechanism of Conventional Articulator and Problems Thereof
FIG. 32 shows the condition when the jaw is moved to the right. The balancing side condyle moves in the anterior, downwards and inwards direction, the amount of its movement being characterized by two angles (planes), namely, the balancing side sagittal condylar path inclination and the angle of balancing side lateral condylar path (Bennett angle).
That is, the vertical characterization of the balancing side condyle is achieved by the balancing side sagittal condylar path inclination, and inner/outer characterization is characterized by means of the Bennett angle.
In contrast, while the working side condyle is pushed outwards due to the Bennett movement (inside/outside characterization is characterized by the Bennett angle of the balancing side condyle), the remaining anterior/posterior characterization and vertical characterization are characterized in the conventional articulator by a rear wall and working side sagittal condylar path inclination plate by adjustment using a screw M in FIG. 7 or a screw in FIG. 32 (the one corresponding to the screw N in FIG. 7). That is, as shown in FIG. 32 or 7, lateral movement is characterized by four plates (four angles).
However, because the movement is not only to be to the right but to left and right, a "full reproduction articulator" is not so easy to achieve. This is because although up to this point only the case where the jaw was moved to the right was considered, the same adjustment as was considered above must now be performed also when the jaw is moved to the left. The working side sagittal condylar path inclination that was employed for vertical characterization when the right condyle was the working side condyle cannot be employed as the balancing side sagittal condylar path inclination when the right condyle now becomes the balancing side condyle. In general, the working side sagittal condylar path inclination and the balancing side sagittal condylar path inclination are different, so it is not possible to represent the vertical characterization of the working side condyle and the sagittal condylar path inclination of the balancing side condyle by a single sagittal condylar path inclination plate. It is not possible to satisfy both the working side and balancing side by a single sagittal condylar path inclination plate. This is because a single plane cannot be used to characterize different movements at the same time.
Consequently, with a conventional articulator, it is necessary to adjust the dentures by adjusting the articulator for the case of mandibular movement to the right side, then to perform adjustment of the articulator once more for the case of mandibular movement to the left side. Since avoiding the problem of excessive time being required for the re-adjustment, conventionally, the sagittal condylar path inclination of the balancing side is also used for the sagittal condylar path inclination of the working side.
Methods of Characterizing the Working/balancing Sagittal Condylar Path Inclination Using a Single Sagittal Condylar Path Inclination Plate
(i) Method of Dividing a Plate of the Sagittal Condylar Path Inclination
In order to overcome this difficulty, a sagittal condylar path inclination plate divided as shown in FIGS. 33a and 33b to provide separate sagittal condylar path inclination plates during working and during balancing respectively might be proposed. However, with this method, if the inclination during working is greater than the inclination during balancing, as shown in FIGS. 33c and 33d, a groove D will be produced which makes smooth movement of the condyle impossible to reproduce. Namely, a sagittal condylar path inclination plate can be used to characterize only one between vertical characterization of the working side condyle or characterization of the amount of descent of the balancing side condyle. Detailed reasons will be given later, but here it will be simply stated that it is the wisest course from the point of view of convenience to employ a sagittal condylar path inclination plate to characterize the amount of descent of the balancing side condyle.
(ii) Method of Rotating the Sagittal Condylar Path Inclination Plate About the Condylar Path Inclination Axis
The following passages deal with a method adopted in an articulator which is currently known as a full-adjustable articulator. As shown in FIG. 7, both the angle of the balancing side sagittal condylar path inclination and the angle of the working side sagittal condylar path inclination can be characterized by a single condylar path inclination plate, according to use of both adjustments of a condylar path inclination plate in two ways, sagittal balancing side condylar path inclination is characterized by rotating the plate about the intercondylar axis as a center axis (N in FIG. 7), and sagittal working side condylar path inclination is characterized by rotating the plate (hereinbelow called the Fischer sliding) about the axis, which is defined by the intersect of the condylar path inclination and sagittal plane, as a central axis (M in FIG. 7).
However, this current method has the following problems.
(7) Points which Require Improvement in the Conventional Articulator
(a) Reproduction of the Fischer Angle
In order to fully reproduce jaw movement, the protrusive movement must also be reproduced. There is a problem in this respect. When the jaw moves protrusively, the left and right condyles move anteriorly and downwards with the inclination of the protrusive sagittal condylar path. As described above, the inclination of the protrusive sagittal condylar path during protrusive movement and the inclination of the sagittal condylar path during lateral movement are different, in general. Consequently, the condylar path inclination plate would have to represent three condylar path inclinations, namely (1) the balancing side condylar path inclination, (2) the working side condylar path inclination and (3) the protrusive condylar path inclination; however, this is impossible. By using the Fischer slide, it is possible to represent the following two inclinations: (1) balancing side condylar path inclination and (2) working side condylar path inclination. But this approach can also be used to represent (1) balancing side condylar path inclination and (3) protrusive condylar path inclination. Two inclinations may be adopted among these three, and the best course is to represent (1) balancing side condylar path inclination and (3) protrusive condylar path inclination. The reason for this is that if (2) the working side condylar path inclination is adjusted after setting (1) the balancing side condylar path inclination, the previously adjusted (1) balancing side condylar path inclination changes. Mutual adjustment interferes with mutual setting. On the other hand, if the protrusive condylar path inclination is set in the first place, and then, the working side condylar path inclination is set by using the screw M (FIG. 7), there is no change of the protrusive condylar path inclination previously set. If this is done, it is necessary that the vertical characterization of the remaining working side condyle should be characterized by a mechanism which is independent from the condylar path inclination plate.
(b) Need of Reproduction of Posterior Movement
On a conventional articulator, posterior movement cannot be performed. In this background, there has been a conventional theory of the "centric relation concept" in which the rear most position of the mandible is the best and beyond which further retrusion was not possible as the starting point. However, in fact, the actual mandible does perform a slight retruded movement. There are many patients of the temporomandibular arthrosis whose cases are thought to be caused by dental prostheses making injurious occlusal contact in retruded movement. On the conventional articulator, retruded checking is impossible. Even dentists could not tell whether there would be early contact on retruded movement, without inserting the dental prosthesis in the oral cavity. It is considered be significant to be able to perform retruded movement on the articulator at the stage of construction of the dental prosthesis and to be able to check whether there is injurious occlusal contact during retruded movement.
(8) Summary
Conventionally, there are various types of full-adjustable articulators, and these have various respective characteristics in detail, but their basic principles are common, enabling the following five elements to be characterized:
(i) Distance Between the Condyles (first element)
(ii) Balancing Side Condyle
(1) Sagittal Condylar Path Inclination Plate (second element)
(2) Bennett Plate (third element)
(ii) Characterization of Working Side Condyle
(3) Rear Wall (fourth element)
(4) Fischer Slide (fifth element) (rotation of sagittal condylar path inclination plate about sagittal condylar path axis)
The above conventional characterizations do not include condyle characterization during protrusive movement. After the condylar path inclination plate has characterized two items, i.e., balancing side condylar path inclination and working side condylar path inclination, the third item of protrusive condylar path inclination cannot be characterized. The characterization of the distance between the condyles, which is the first of the above five elements, has no relationship with the characterization of condyle movement. The above five elements are therefore essentially only four. The characterization of the intercondylar distance is only useful in characterizing the center point of rotation of the lateral movement.
That is to say, full reproduction of condyle movement cannot be achieved with the above four characterizations. By this omission of the remaining one of the characterizations, protrusive movement cannot be reproduced because the characterization of the protrusive condylar path inclination is lost. In actual practice, the protrusive characterization is adjusted by partial grinding or adding thickness of the plate of sagittal condylar path inclination. The reason for the necessity for grinding or adding is that the Fischer slide is used for vertical characterization of the working side condyle.
As described above, even conventional articulators known as full-adjustable articulators, cannot, in fact, reproduce protrusive movement of the jaw. Since today's dental prostheses constructed on the conventional semi-adjustable articulators do not harmonize with the actual chewing movement of mandible at all, they often give an uncomfortable sensation. From the view of the above clinical circumstances, an object of the present invention is to provide a full reproduction articulator which is capable of reproducing faithfully and accurately the entire jaw movement, in particular the articulation, including individual differences.
As another object of present invention, this articulator should be provided not only for faithful and accurate reproduction of jaw movement, but also for clinical means of treatment. In cases where faithful and accurate reproduction would cause injury to the body, the articulator must be capable of providing new ideal mandibular positions and new ideal pattern of jaw movement.
Namely, by means of a remount function, the mandibular plaster model can be transferred to an ideal optimum position on the articulator from a position mounted faithfully and accurately just as in the current condition of the mandible, and if necessary, can be returned at any time to the previous actual position from the optimum position.